Baird T-18C Restoration

The cabinet of this set is in horrible condition. It has obviously
been sitting in someone's storage shed for years, and has been
damaged by water. The CRT has been damaged and is probably full
of air. It appears that a postwar tube will fit perfectly, and will
work with minor modification to the set.

There is also a strange pilot lamp mounted inside the cabinet near
the CRT. It illuminates the rear of the CRT screen with an orange
glow when the radio is in use. Here is technical
information on this set.

Cabinet. (pictures) The cabinet will require a full restoration. Perhaps
some of the original veneer can be salvaged, but most will have to be
replaced. There doesn't appear to be worm damage, however.
Restoration of the cabinet is complete.

Chassis. (pictures) There are two chassis in this set, the Radio/Power
Supply Chassis and the TV Chassis. These chassis have a
considerable amount of rust on them. Each chassis will be cleaned
with water and a mild detergent using soft brushes to get into small
places. We will attempt to clean the rust off the chassis, and have
the detachable parts replated. Then, all paper capacitors will be
replaced with modern ones (see the procedure for this). Each electrolytic capacitor will be tested for leakage and
capacity. If bad, new electrolytics will be installed
inside the old ones. The high voltage (EHT) capacitor also had
to be rebuilt.

We have no technical information on this set, so we will have to
trace the circuit diagram.

After inspecting the chassis, I determined that the rust was too
severe to be removed. So, all of the components have been stripped
off the chassis, and they will be taken to be replated.

The chassis have been replated.

Radio/Power Supply Chassis: This
chassis has the B+ (HT) and high voltage (EHT) power supplies,
and the radio circuitry. The tubes (valves) in the radio section are
TH4B (triode-hexode first detector), VP4B (IF amplifier), 2D4A
(detector), SP4B (audio amplifier) and AC4PEN (audio output).The B+
(HT) rectifier is apparently a UU5 and the high voltage (EHT)
rectifier is apparently a HVR2.

I have reassembled the Radio/Power Supply chassis. Since we have no
schematic diagram, I have been tracing the circuit as I go along.

There are a couple of unusual features. First, it appears that the
filament current for the CRT was critical, and the set has a set
of jacks to adjust it. Second, the audio amplifer (SP4B) also
serves as a 41.5 mHz preamplifer. The sound RF is sent from the TV
chassis. I am not certain yet whether it is further amplified at 41.5
and then detected, or if it is down converted to the radio IF frequency.

The 41.5 mHz TV sound is down converted to 456 kHz in the frequency
changer. The TV sound works fine now, but the radio doesn't. The
local oscillator works on all three bands, but there is no IF output.
The problem may be in the antenna coils, which I may have connected
improperly. I found a couple of schematics of Bush radios from that
era (Bush made Baird TV sets before the war) that look similar. That
will help find the problem. Since I haven't yet rebuilt most of the
paper capacitors, one or more of them may be bad.

The problem was a couple of wiring mistakes I made when reassembling
the chassis. It now works perfectly.

The speaker field coil winding is open. I dissassembled it and could
find no obvious fault, so rewinding was the only option. Using a jig
made with an electric drill I wound over 3000 feet of 30 ga. wire
onto the old form.

TV Chassis: The other
chassis has a brass RF section with tubes (valves) inside. This
contains the RF amplifiers and detector. On the chassis there is a
AC2/HL valve, which is the sync amplifier, and five tubes (valves)
used in the sweep section. The horizontal (line) sweep uses a 2D4A
diode, a AC/P output tube, and a UR1C damper. The vertical (frame)
sweep uses another 2D4A diode and a AC/P oscillator/output. A strange transformer is used in this set. It has an extended frame which comes up around
the picture tube. This transformer serves as both the vertical
(frame) output transformer and deflection coil.

I have reassembled the sweep portion of the TV
chassis. I have a schematic diagram of a similar set's sweep
circuits, so tracing this set's design was easy. I have rebuilt the
electrolytic capacitors, and now must rebuild the paper ones. The
paper capacitors have been replaced, and the horizontal (line) and
vertical (frame) sweep circuits appear to work properly.

I have now started on the RF module.
It contains three TSE4 tubes (valves), two as RF amplifiers and one
as a video amplifier, plus a diode detector. The RF module now works.

I have now tested the entire set, and everything appears to work. I
am missing the horizontal (line) oscillator tube (valve), and I am
attempting to have the CRT (Baird
Cathovisor) rebuilt.

I now have the tube (valve). I have installed a Mullard MW 31-18
postwar CRT in the set. It is physically about the same size, but has
a very different gun. The original tube has the cathode tied directly
to the filament internally, and is a triode. The filament operates on
2 volts. The MW 31-18 has a 7.5 volt filament, a separate cathode
lead, and is a tetrode. To get the required filament voltage, I
connected the 2 volt supply in series with the 4 volt supply used for
the other tubes (valves) in the set. There is a plug/socket assemly
to match the CRT filament current. By using the lowest setting I was
able to get 6.3 volts.

I connected grid #2 of the new tube to a source of about 150 volts in
the set, and slightly changed the brightness circuit. I now have an
excellent, stable picture.

The MW 31-18 is about an inch longer than the original tube, so the
chassis sticks out of the rear of the cabinet. I am hoping that the
original tube can be rebuilt.